Story progression

Story progression is a very useful, but unpredictable game option in The Sims 3 and is a much more powerful version of free will. It helps keep a town alive and balanced.

With story progression on, uncontrolled Sims' lives progress normally: Neighbors may move away, new ones will move in, get promotions, get married, have children, make enemies, and even die. In this way, it adds a true sense of realism to the game, enhancing the gameplay experience in incredible, surprising ways.

Although it is seen by some as efficient and clever, (and the game recommends that it be enabled) story progression can sometimes cause unwanted and at times highly chaotic events to occur in the player's utopia, as it is very random in what it decides; the player may return to their other households to find their families in many sudden situations that they may find difficult to reverse. Story progression may result in unwanted marriages/divorces, absurd relationship changes (such as best friends becoming rivals, or an adult becoming friends with a baby), new members to the household or members that have curiously moved away, job losses, etc. As the game recommends that story progression should be enabled, many players are uncertain of its changes, most of whom have fallen victim to its problems and have become frustrated with its occasional disastrous effects. Strictly speaking, story progression should only be enabled if the player is fully sure that they do not mind inactive Sims running their own lives and making their own choices, even if it may inadvertently result in hard work gone to waste.
[1] Story progression is also disastrous in case aging is set to off, because Sims will be able to have babies who don't age and the household will be filled up with unwanted babies in no time. However, it can be disabled if the player wishes to.

Many players find that EA's story progression make very little progresses throughout the town, such as very few newborns as players play for longer generations. EA's story progression is also notorious for emigrating random Sims at any time whenever the town is considered to be overpopulated, making player's Sims to lose their friends randomly without warning.